1. Field of the Invention
The invention generally relates to the field of sound reproduction by means of loudspeakers, also named electro-dynamic or electro-acoustic transducers, which convert an electrical energy generally delivered by an amplifier into acoustical energy.
2. Description of the Related Art
Acoustical energy is radiated through a diaphragm the displacements of which induce variations of pressure of the ambient air, which propagate in space under the form of an acoustic wave.
In the Rice-Kellog type electro-dynamic transducer, which is the most common, the diaphragm is driven by a movable coil including a solenoid in which passes an electric current (from the amplifier) and which dives into an air gap filled with a magnetic field produced by a permanent magnet. Interaction between the electric current and the magnetic field induces a force known as the Laplace force driving the movable coil, which in turn drives the diaphragm, the vibrations of which produce an acoustic radiation.
Numerous designs were imagined for diaphragms; most common shapes are a cone (the generatrix of which may be straight or curved) and a dome, or a combination thereof.
In the case of the cone, the movable coil is generally fixed onto the edge of an opening formed in the center of the diaphragm. The size and mass of the moving part are somewhat important, reason for which such architecture is especially adapted to the manufacture of transducers designed for the reproduction of low range and mid range frequencies, requiring diaphragm vibrations of low frequency and great amplitude.
In the case of the dome, the movable coil is generally fixed to a peripheral edge of the diaphragm. The size and mass of the moving part may be minimize, reason for which such architecture is especially adapted to the manufacture of transducer designed to reproduce of high range, due to diaphragm vibrations of high frequency and low amplitude.
Whichever its shape, the diaphragm is generally fixed to a chassis of the transducer trough a peripheral suspension which, in addition to its primary function of holding the diaphragm, has three general functions:                return effect to the diaphragm toward a rest position,        producing a secondary acoustic radiation which adds to the radiation of the diaphragm.        centering and axial guiding of the moving part (including the diaphragm and the movable coil) with respect of the air gap,        
In cone diaphragm transducers, due to great displacements, the suspension is generally not sufficient to guide the diaphragm with respect of the air gap. This is which complementary centering devices are generally provided, like of the spider type (Cf. French patent application FR 2 667 212 in the name of the applicant).
In the case of dome shaped diaphragms, the displacements of which are far smaller, a sole peripheral suspension is generally provided to ensure all three functions discussed hereinbefore. Such a topology has been known for a long time, Cf. U.S. Pat. No. 2,242,791 (Edward C. Wente/Bell Laboratories) of June 1948. A more recent example is exposed in US patent application No. US 2008/0166010 (Stiles et al).
It is known that axial guiding and centering of the diaphragm with respect of the air gap are an essential function of the suspension. Indeed, it is necessary to avoid (or at least minimize) the transversal movements (swinging, pitch) of the diaphragm, considered as defects which generate distortions within the emitted sound signal. In particular, the coil may rub on the walls of the air gap. Such a rubbing induces strong distortions and parasite noises which prevent use of the transducer.
This is why the centering of the moving part with respect of the air gap is a tricky assembly operation, which requires taking into account all manufacturing clearances (in particular of the magnetic circuit) and also requires a very precise fixation of the suspension on the transducer chassis. Such an operation is difficult to automate. Despite all precautions, rubbing of the moving coil on a side wall of the air gap may arise and it is usual, in order to minimize such occurrence, to provide between the moving coil and the air gap important internal and external operational clearances, of several tenths of millimeter.
However, enlarging the air gap has harmful consequences:                it decreases, in a same magnetic circuit, the density of magnetic flow within the air gap, which decreases in proportion the driving force provided to the moving coil and thereby decreases the efficiency of the transducer,        it decreases the capability of dissipating heat produced by Joule effect within the coil, due to the thickness of air layers which surround the coil and act as thermal insulators.        
Part of the efforts made by the loudspeaker manufacturers is directed toward the research of the best compromise between centering clearances of the moving part with respect of the air gap (and hence suspension dimensioning and/or fixing clearances), and the acoustic performances of the transducer. As already stated, increasing the clearances decreases the performances. Of course, in an industrial manufacturing environment, the choice is generally directed to an increase of clearances, and a decrease of acoustical performances.
In order to address this problem, the applicant has made the opposite choice, in order not to scarify performances and to search for pertinent and rational solutions in the transducer architecture.
The invention therefore aims at proposing a solution to the problems disclosed hereinbefore, in particular for high range transducers, and at providing improvements to dome diaphragms in order to facilitate the assembly thereof without sacrificing the acoustical performances.